In electronic circuits, devices in the forms of bipolar junction transistor (BJT), heterojunction bipolar transistor (HBT), metal semiconductor field effect transistor (MESFET), metal oxide semiconductor field effect transistor (MOSFET), pseudomorphic high electron mobility field effect transistor (pHEMT) and metamorphic high electron mobility field effect transistor (mHEMT) are used for switching and amplification, whereas devices in the forms of p-n junction and Schottky junction are used as rectifiers. The semiconductors for the above devices may be selected from a group of Si, Si/Ge, GaAs, InGaAs, GaN, InGaN, AlInN and etc. For high power applications, the above-mentioned devices are operated at high current densities and the un-wanted joule heating in the semiconductor regions and interfaces between the semiconductor and metal electrodes is high. The un-wanted joule heating leads to an increase in the temperature of the devices and circuits. Take GaN HEMTs as an example, the local channel temperature can be as high as 500-1000 K at a bias voltage of 20 V. In addition to the variation of temperatures during the operation, the devices and circuits may go through heating process during the fabrication and after the metal electrodes have been deposited.
It is noted that for the formation of metal electrodes, the selection of materials is determined by the following considerations: [1] low electrode resistance, [2] good adhesion, [3] low thermal diffusion, and [4] good stability. Due to these requirements, materials for forming metal electrodes in electronic devices are limited. For III-V compounds, various metals such as Ti, Al, Au, Ge, Ni, Pd, Zn, W and their combinations have been developed for both ohmic electrodes and Schottky electrodes. For Si based devices and circuits, materials for metal electrodes are often selected from a group of W, WSi2, Ti, TiN, Cu, Al, TaSi2, TiSi2, etc.
As shown in Table 1, although the above-described materials are suitable for forming low resistance electrodes to the semiconductors listed, their thermal expansion coefficients are substantially larger than that of the semiconductors in the list. Therefore, during the fabrication or operation when the temperatures are raised, there is more severe expansion of the metal electrodes on the semiconductor substrate than that of the semiconductor, causing tensile stresses in the semiconductors and the interfaces. Contrarily, when the temperatures are reduced, the shrinkage of the metal electrodes is more severe than that of the semiconductors, causing a compressive stresses in the semiconductors and the interfaces. Microscopic defects are often formed in semiconductors due to these strain and stresses. The formation of these microscopic defects may lead to performance degradation and lifetime reduction in the devices.
TABLE 1Thermal expansion coefficients of some materialsCoefficientCoefficientMaterial(×10−6/K)Material(×10−6/K)GaAs5.73Au14.1InGaAs5.05Cu17InP4.6Ti9Ge5.6Ni13Si3.2Al23 (12.9)GaN4AlN4.5InN3Alumina6-7Sapphire4.5Silicon carbide3.2Epoxy Resin10-100
For power devices operated at high frequencies, the power density can not be easily reduced due to the limitation of phase delay. A typical power density is 1 W per mm length of the metal electrodes for GaAs-based devices. Such high power density will cause elevated temperatures in the channel layers. Take HEMTs for power application as an example, the GaAs-based semiconductor channel temperature can rise to 150° C. whereas the ones based on GaN can be more than 200° C. Formation of microscopic defects in the semiconductor channels caused by strain and stresses is therefore inevitable during fabrication and operation, especially for power devices.